1. Field of the Invention
The present invention relates to an electrostatic chuck.
2. Description of Related Art
In the processes of semiconductor device fabrication, a surface of a semiconductor wafer as a substrate undergoes various processes such as deposition, heating and etching. When these processes are performed, an electrostatic chuck is used to hold the semiconductor wafer. This electrostatic chuck holds the semiconductor wafer by generating electrostatic force between a holding face of the electrostatic chuck and the semiconductor wafer set on the holding face. Electrostatic chucks have advantages of fewer limitations on usable environments and smaller risks of particles generation and wafer pollution than those devices which hold a semiconductor wafer with vacuum suction or those devices which mechanically hold a semiconductor wafer.
An electrostatic chuck generally includes a ceramic base in which an electrode for causing electrostatic force is embedded in vicinity to the holding face for holding a substrate. This electrode in the ceramic base is connected to a terminal inserted through an introduction hole formed in a back face, which is the reverse side of the holding face. Electric power is externally supplied to this terminal, whereby electrostatic force is caused on the holding face of the ceramic base. Such an electrostatic chuck is described in, for example, the specification of Japanese Patent No. 2836986.
In the course of semiconductor device fabrication, when various processes are carried out on a semiconductor substrate held onto an electrostatic chuck, this semiconductor substrate needs to be kept at a given temperature. To this end, a temperature control member, such as a heater or cooling jacket, is disposed in contact with the back face of the electrostatic chuck, and the temperature of the surface of the semiconductor substrate held onto this electrostatic chuck is controlled to be constant by the heat transfer across this temperature control member and the electrostatic chuck. This temperature control member has a penetrating hole through which the above-mentioned terminal to be connected to the electrostatic chuck is inserted.
For this temperature control member, a metal member is used in many cases, with the intention of accomplishing favorable heat transfer to the electrostatic chuck. This metal temperature control member needs to be electrically insulated from the terminal, which is attached to the back face of the electrostatic chuck and is supplied with electric power. Therefore, in the penetrating hole, formed in the temperature control member to allow the terminal to pass therethrough, and its vicinity, an insulating member is provided so as to cover the terminal.
One of the properties required of the insulating member interposed between the terminal and the temperature control member is an insulating property between the terminal and the temperature control member. To cause desired electrostatic force on the holding face of the ceramic base, the terminal is supplied with a voltage of, for example, approximately 500 V in the case of a Johnson-Rahbeck-type electrostatic chuck, or approximately 3000 V in the case of a Coulomb-type electrostatic chuck. The insulating member is required to be capable of sufficiently insulating the temperature control member from the terminal, which is supplied with such a voltage.
In recent years, for more reliable and stable insulation, a need has arisen to increase the insulating performance by means of an insulating member. Here, to accomplish high insulating performance between the terminal and the temperature control member, it is necessary that the dielectric voltage of the insulating member itself be high, and that no creeping discharge occur in an area where an end face of the insulating member is in contact with the back face of the ceramic base. To meet the former need to increase the dielectric voltage of the insulating member itself, it is effective to use a material with high dielectric voltage for the insulating member, or to increase the thickness of the insulating member. To meet the later need to prevent the occurrence of a creeping discharge, it is effective to secure a sufficient creepage distance.
However, to thicken the insulating member or to increase the creepage distance, it is necessary to widen the diameter of the penetrating hole, which is formed in the temperature control member, through which the terminal is inserted, and on which the insulating member is provided. If the diameter of this penetrating hole is made larger, a part of the ceramic base facing this penetrating hole is not in contact with the temperature control member, with the result that the heating temperature in this part is locally different from the heating temperature in the other part. Accordingly, the uniformity of temperature of a wafer held onto the electrostatic chuck is degraded. As a result, for example, the etching rate and etching shape in the etching process are degraded, leading to the possibility that a device failure might occur in the processed wafer.
On the other hand, if the above-mentioned diameter of the penetrating hole is made smaller, degradation in the uniformity of wafer temperature as described above does not occur, but the thickness of the insulating member interposed between the terminal and the temperature control member, or the creepage distance becomes insufficient, leading to the possibility that an insulation failure might occur.
Therefore, according to the conventional techniques, it has been difficult to achieve good characteristics both in the uniformity of wafer temperature and in the insulating performance of an insulating member.
The present invention is made to advantageously solve the above described problems, and an object of the present invention is to provide an electrostatic chuck that can obtain good characteristics both in the uniformity of wafer temperature and in the insulating performance of an insulating member.